Spinal fusion surgical procedures involve rigidly fixing two or more vertebrae together in order to alleviate back pain or correct spinal malalignments or degenerative conditions. FIG. 1A shows a top or superior view 10 of a vertebra 1 comprising a body 2, the spinal cord 3, spinous process 4, transverse process 5 and vertebral arch 6 comprising lamina 7 and pedicle 8. FIG. 1B further shows a left posterolateral view 11 of articulated vertebrae 1 each separated by intervertebral disc 9. Spinal fusion surgery normally involves insertion of a rigid orthopaedic structural implant, which may also be referred to as an orthopaedic device or support, and laying bone graft material around the implants in order to promote subsequent growth of a rigid bone graft (or bridge) to permanently fix the two or more vertebrae together. The rigid structural implants may be left in place, or be removed at a later date once the rigid bone graft has grown.
There are two main types of spinal fusion surgery which differ in the location/position of the structural implants and location of bone graft material. In an interbody spine fusion the bone graft is placed in between the vertebral bodies where the intervertebral disc 9 usually lies. The intervertebral disc 9 has to be partially cut away or completely removed and endplates cleaned prior to placement of a structural support and the graft material. This will allow the fusion to occur from one vertebral body to the other through their endplates (ie from one body 2 to the next body 2). In a posterolateral fusion, the bone graft links the transverse process 6 of one vertebra to the transverse process of the next vertebra. The most common fixation technique employed is pedicle screw fixation. In this procedure the surgeon makes an incision in the midline of the back and the back muscles surrounding the spine are then retracted to the side to expose the posterolateral area and to create a bed for the bone graft. Traditionally the area between the adjacent transverse processes has been an ideal place to achieve bony union between vertebra. Screws are placed within the pedicles of adjacent vertebral segments and are then connected with a metal rod (typically titanium, or a titanium alloy). This is performed on one side of the spine or both sides of the spine (ie bilaterally) and may span two or more vertebra. For example a one level bilateral fusion would use four screws and two rods to fuse two vertebrae and a two level bilateral fusion would use six screws and two rods to fuses three vertebrae. FIG. 2A is a perspective view of an illustrative example of a one level pedicle screw and rod arrangement 20. In this example each pedicle screw 21 comprises a threaded shaft 22 attached to a crown 24, and a lock screw 26. The shaft 22 and crown 24 are provided as a single component (ie parts may be manufactured individually and then assembled into a single component part) in which the crown comprises a cavity and an aperture in its base through which shaft 22 passes. The upper end of the shaft 22 ends in a ball 23 with a diameter larger than the aperture to retain and link the shaft 22 to the crown 24. This allows relative movement of the shaft with respect to the crown. The crown further comprises a U shaped saddle 25 to receive a rod 28 which is used to link adjacent pedicle screws (and thus vertebra). The depth of the saddle cut-out rod 23 is such that the rod 28 is located above the ball 23. A locking screw 26 is inserted into the top of the crown to lock the rod in place. This arrangement can be installed in parts (eg shaft 22 and crown 24 component, then rod 28, and then locking screw 26), and allows flexible installation as each pedicle screw 21 can be orientated at a different angle to the other pedicle screws 21. Other variants of pedicle screw systems may use variants (for example the shaft and crown may be manufactured as a single part with the shaft in a fixed orientation with respect to the crown. Bone graft promoting materials, either taken from the patient, a cadaver or synthetically generated (or some combination of the three sources) is laid out on the exposed posterolateral area and around the rods and screws, and the muscles are laid back down over the graft and the incision closed. The bone graft promoting materials include bone fragments, tissue or similar material, and or a solution containing bone morphogenic protein (BMP) or materials soaked or infused with BMP. FIG. 2B is a top view of an open surgery two level fusion 210 illustrating the laying of bone graft material 15 around the rods 28. In some cases a laminectomy may also be performed in which a portion of the lamina is removed to expose the nerves and spinal cord 3. Over the next few months, the bone graft material 15 and adjacent transverse processes 5 and vertebral bodies 2 will gradually fuse into a single rigid bone structure.
The treatment of spinal conditions with Minimally Invasive Surgery (MIS) has, or is rapidly becoming, the preferred method due to the reduced risk of complications and reduced recovery time. Some studies of MIS Spine surgery have reported benefits including reduced blood loss, less infection, less post-operative pain and reduced hospital stays, benefits which also enable complex surgery to be performed on older and sicker patients.
However up until recently posterolateral fusion has only been possible through open surgery where trauma may occur to tissue surrounding the vertebrae. In a MIS posterolateral fusion, case stab incisions are made only above the screw incision points, and the linking rod is pushed through the tissue between the incision points. That is, tissue surrounding the rod and between the two pedicle screws is not exposed, and thus bone graft promoting material cannot be delivered along the path along which the bone growth is desired. Thus in prior attempts at MIS posterolateral fusion procedures generally no attempt is made for bony fusion. This may lead to instrumentation failure and pseudoarthrosis, and removal of instrumentation. There is thus a need to develop devices to enable MIS for posterolateral fusion, or to at least provide surgeons with a useful alternative to current surgical methods for posterolateral fusion.